Bedform patterns, subglacial meltwater events, and Late Devensian ice sheet dynamics in north-central Ireland     

Jasper Knight   《Global and Planetary Change》2003,35(3-4):237-253

Glacial bedform patterns and sediments deposited by the temperate and polythermal Late Devensian ice sheet in north-central Ireland record changes in the processes, location, and magnitude of subglacial meltwater throughout the last full glacial cycle (21–14 ¹⁴C kyear BP). Meltwater characteristics are related directly to basal ice thermal regime and ice dynamics, including ice velocity and shifts in the location of ice centres. Therefore, reconstructed meltwater characteristics may provide insight into wider controls on dynamic ice behaviour. A range of meltwater-related features are present across north-central Ireland. These include tunnel valleys, drumlin leeside sequences, eskers, and boulder lags. Other bedforms including Rogen moraines were modified by meltwater activity along ice streams. Meltwater was stored subglacially in two contrasting regions located beneath or near ice centres in north-central Ireland. (1) The Lough Erne Basin is developed in a lowland depression occupied partly by subglacial Rogen moraine ridges which were formed around the time of the last glacial maximum. Meltwater was stored between Rogen ridge crests and released by hydraulic jacking associated with drumlinisation (16.6 ¹⁴C kyear BP) and ice streaming (13.8 ¹⁴C kyear BP). (2) The Lough Neagh Basin occupies a similar lowland depression and was the location of an ice sheet centre throughout the last glacial cycle. No bedforms are present beneath or immediately surrounding Lough Neagh. A larger, more continuous meltwater lake existed in the Lough Neagh depression, probably sealed by a region of cold-based ice outside lake margins. Water escaped through regional-scale tunnel valleys, particularly the Poyntzpass channel which was active during the Carlingford ice readvance (Killard Stadial, correlated with Heinrich event 1 at 14.5 ¹⁴C kyear BP). Overall, reconstructed subglacial lake characteristics and drainage mechanisms are related closely to basal ice thermal regime and substrate relief (controlling lake geometry), and provide insight into controls on overall ice sheet dynamics.  相似文献   

Rapid climate change     

Jasper Knight   《Global and Planetary Change》2006,54(3-4):209-210

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The magnetic field draping direction at Mars from April 1999 through August 2004     

David A. Brain  David L. Mitchell  Jasper S. Halekas 《Icarus》2006,182(2):464-473

Using more than five years of data from the magnetometer and electron reflectometer (MAG/ER) on Mars Global Surveyor (MGS), we derive the draping direction of the magnetic field above a given latitude band in the northern hemisphere. The draping direction varies on timescales associated with the orbital period of Mars and with the solar rotation period. We find that there is a strongly preferred draping direction when Mars is in one solar wind sector, but the opposite direction is not preferred as strongly for the other solar wind sector. This asymmetry occurs at or below the magnetic pileup boundary (MPB), is observed preferentially on field lines that connect to the collisional ionosphere, and is independent of planetary longitude. The observations could be explained by a hemispherical asymmetry in the access of field lines to the low-altitude ionosphere, or possibly from global modification of the low-altitude solar wind interaction by crustal magnetic fields. We show that the draping direction affects both the penetration of sheath plasma to 400 km altitudes on the martian dayside and the radial component of the magnetic field on the planetary night side.  相似文献   

Reply to billings concerning ‘Two-Fluid Model of the Solar Corona’     

J. W. Knight  C. E. Newman  P. A. Sturrock 《Solar physics》1975,41(2):371-371

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Periodic drainage of ice-dammed lakes as a result of variations in glacier velocity     

Peter G. Knight  Fiona S. Tweed 《水文研究》1991,5(2):175-184

Previous discussions of the catastrophic drainage of ice-dammed lakes have centred on mechanisms where characteristics of the lake are crucial to drainage initiation, for example dam flotation or tunnel formation at a critical lake depth. This paper describes a mechanism for lake drainage where drainage initiation depends on the characteristics of the glacier and is independent of the characteristics of the lake. Prediction of this mechanism must be based on glacier dynamics, whereas the mechanisms most commonly discussed previously are best predicted primarily on the basis of lake evolution. An ice-dammed lake at the margin of the glacier Solheimajokull, in southern Iceland, was observed to drain rapidly into the sub- or englacial drainage system, supplying water and debris to the bed or interior of the glacier. Geomorphological evidence suggests that the lake drains and refills periodically, discharging up to 13300 m³ of water into the glacier-hydrological system. The depth of the maximum lake is insufficient to cause either flotation of the ice margin or tunnel opening by plastic deformation of the ice, and we suggest that sudden drainage is related to ice-bed separations associated with specific glacier flow states rather than to a critical lake depth threshold. This mechanism of lake drainage has implications for conditions at the glacier bed, for the development of basal ice and for the entrainment of debris into the glacier, as well as for the prediction of potentially hazardous catastrophic drainage events and jokulhlaups from ice-dammed lakes.  相似文献   

Asteroseismology of the β Cephei star ν Eridani – I. Photometric observations and pulsational frequency analysis     

G. Handler  R. R. Shobbrook †  M. Jerzykiewicz  K. Krisciunas  T. Tshenye  E. Rodríguez  V. Costa  A.-Y. Zhou  R. Medupe  W. M. Phorah  R. Garrido  P. J. Amado  M. Paparó  D. Zsuffa  L. Ramokgali  R. Crowe  N. Purves  R. Avila  R. Knight  E. Brassfield  P. M. Kilmartin  P. L. Cottrell 《Monthly notices of the Royal Astronomical Society》2004,347(2):454-462

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Improving Groundwater Model Calibration with Repeat Microgravity Measurements     

Jeffrey R. Kennedy  Libby Wildermuth  Jacob E. Knight  Joshua Larsen 《Ground water》2022,60(3):393-403

Groundwater-flow models depend on hydraulic head and flux observations for evaluation and calibration. A different type of observation—change in storage measured using repeat microgravity—can also be used for parameter estimation by simulating the expected change in gravity from a groundwater model and including the observation misfit in the objective function. The method is demonstrated using new software linked to MODFLOW input and output files and field data from the vicinity of the All American Canal in southeast California, USA. Over a 10-year period following lining of the previously highly permeable canal with concrete, gravity decreased by over 100 μGal (equivalent to about 2.5 m of free-standing water) at some locations as seepage decreased and the remnant groundwater mound dissipated into the aquifer or was removed by groundwater pumping. Simulated gravity from a MODFLOW model closely matched observations, and repeat microgravity data proved useful for constraining both hydraulic conductivity and specific yield estimates. Specific yield estimated using the infinite-horizontal slab approximation agreed well with model-derived values, and the departure from the linear, flat-water-table approximation was small, less than 2%, despite relatively large and dynamic water-table slope. First-order second-moment parameter uncertainty analysis shows reduction in uncertainty for all hydraulic conductivity and specific yield parameter estimates with the addition of repeat microgravity data, as compared to drawdown data alone.  相似文献   

Apportioning deformation among depth intervals in an aquifer system using InSAR and head data     

Smith  Ryan G.  Hashemi  Hossein  Chen  Jingyi  Knight  Rosemary 《Hydrogeology Journal》2021,29(7):2475-2486

Land surface subsidence due to excessive groundwater pumping is an increasing concern in California, USA. Interferometric Synthetic Aperture Radar (InSAR) is a remote sensing technique for measuring centimeter-to-millimeter surface deformation at 10–100 m spatial resolution. Here, a data-driven approach that attributes deformation to individual depth intervals within an aquifer system by integrating head data acquired from each of three screened intervals in a monitoring well with InSAR surface deformation measurements was developed. The study area was the Colusa Basin in northern Central Valley. To reconstruct the surface deformation history over the study area, 13 ALOS-PALSAR scenes acquired between 2006 and 2010 were processed. Up to ~3-cm year^?1 long-term subsidence and up to ~6 cm seasonal subsidence were observed using the InSAR technique. The technique developed in this paper integrates the InSAR-observed seasonal deformation rate and the co-located head measurements in multiple depth intervals to estimate the elastic skeletal storage coefficient, the time delay between the head change and the observed deformation, and subsequently the deformation of each depth interval. This technique can be implemented when hydraulic head measurements within each depth interval are not correlated with each other. Using this approach, the depth interval that contributed the most to the total subsidence, as well as storage parameters for all intervals, are estimated. The technique can be used for identification of the depth interval within the aquifer system responsible for deformation.

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The Cretaceous/Paleogene (K‐Pg) boundary at the J Anomaly Ridge,Newfoundland (IODP Expedition 342, Hole U1403B)          下载免费PDF全文

Dominik Loroch  Alexander Deutsch  Jasper Berndt  André Bornemann 《Meteoritics & planetary science》2016,51(7):1370-1385

We present results of an in‐situ geochemical study using laser‐ablation inductively coupled plasma–mass spectrometry (LA‐ICP‐MS) analyses along a ~4.3 cm long section across the K‐Pg event bed, drilled during IODP Expedition 342 at J Anomaly Ridge south of St. John's, Newfoundland. This section comprises the Maastrichtian with a sharp boundary to the graded, between 1.5 and 1.8 cm thick ejecta layer with totally altered impact glass spherules, which in turn is topped by Danian sediments. The porous and clayey material required elaborate preparation in order to yield reliable data. The ejecta bed shows a highly variable depletion in rare earth elements that even results in strongly subchondritic concentrations. The Ce/Ce* varies strongly (0.81–34), Ni/Cr ranges from 0.38 to 2.79. The maximum platinum group elements (PGE) concentrations are located in one LA‐spot exactly at the basis of the ejecta layer; they amount (in μg g^?1) to 0.35 (Rh), 1.64 (Pd), 2.79 (Pt), and 0.86 (Au). The Nb/Ta ratio increases in the Ma from ~10 to 35.9 toward the ejecta horizon, which itself has higher Nb, Ta, Zr, and Hf concentrations than the background sedimentation, combined with low Nb/Ta (~5–10), and low Zr/Hf (~20–30). The overall result is that alteration processes changed totally the original geochemical characteristics of this K‐Pg spherule bed. To explain the exorbitant element mobility at distances of hundreds of μm, we discuss a combination of mostly reducing redox processes and interaction with organic compounds. This study demonstrates the high potential of in‐situ analyses with high spatial resolution at complex geological materials. Moreover, our results indicate that some caution is necessary in determining the projectile type in impactites via PGE ratios.  相似文献   

A comparison of the sedimentary records of the 1960 and 2010 great Chilean earthquakes in 17 lakes: Implications for quantitative lacustrine palaeoseismology          下载免费PDF全文

Maarten Van Daele  Jasper Moernaut  Lindsey Doom  Evelien Boes  Karen Fontijn  Katrien Heirman  Willem Vandoorne  Dierk Hebbeln  Mario Pino  Roberto Urrutia  Robert Brümmer  Marc De Batist 《Sedimentology》2015,62(5):1466-1496

Seismically‐induced event deposits embedded in the sedimentary infill of lacustrine basins are highly useful for palaeoseismic reconstructions. Recent, well‐documented, great megathrust earthquakes provide an ideal opportunity to calibrate seismically‐induced event deposits for lakes with different characteristics and located in different settings. This study used 107 short sediment cores to investigate the sedimentary impact of the 1960 M_w 9·5 Valdivia and the 2010 M_w 8·8 Maule earthquakes in 17 lakes in South‐Central Chile (i.e. lakes Negra, Lo Encañado, Aculeo, Vichuquén, Laja, Villarrica, Calafquén, Pullinque, Pellaifa, Panguipulli, Neltume, Riñihue, Ranco, Maihue, Puyehue, Rupanco and Llanquihue). A combination of image analysis, magnetic susceptibility and grain‐size analysis allows identification of five types of seismically‐induced event deposits: (i) mass‐transport deposits; (ii) in situ deformations; (iii) lacustrine turbidites with a composition similar to the hemipelagic background sediments (lacustrine turbidites type 1); (iv) lacustrine turbidites with a composition different from the background sediments (lacustrine turbidites type 2) and (v) megaturbidites. These seismically‐induced event deposits were compared to local seismic intensities of the causative earthquakes, eyewitness reports, post‐earthquake observations, and vegetation and geomorphology of the catchment and the lake. Megaturbidites occur where lake seiches took place. Lacustrine turbidites type 2 can be the result of: (i) local near‐shore mass wasting; (ii) delta collapse; (iii) onshore landslides; (iv) debris flows or mudflows; or (v) fluvial reworking of landslide debris. On the contrary, lacustrine turbidites type 1 are the result of shallow mass wasting on sublacustrine slopes covered by hemipelagic sediments. Due to their more constrained origin, lacustrine turbidites type 1 are the most reliable type of seismically‐induced event deposits in quantitative palaeoseismology, because they are almost exclusively triggered by earthquake shaking. Moreover, they most sensitively record varying seismic shaking intensities. The number of lacustrine turbidites type 1 linearly increases with increasing seismic intensity, starting with no lacustrine turbidites type 1 at intensities between V½ and VI and reaching 100% when intensities are higher than VII½. Combining different types of seismically‐induced event deposits allows the reconstruction of the complete impact of an earthquake.  相似文献